Spectral reflectivity is the fraction of light reflected by an object. This fraction typically varies with the wavelength of light incident on the object. Spectral reflectivity can be used to characterize the surface of an object. Digital cameras can be used to estimate the spectral reflectance of a surface. However, digital cameras are not optimized for measuring spectral reflectance or for quantitative color measurements.
Most digital cameras capture digital color images in a trichromatic fashion using three distinct sets of detectors. The detectors may, for example, be complementary metal-oxide semiconductor (“CMOS”) or charge-coupled device (“CCD”) sensors. Each set of detectors is sensitive to different wavelengths (colors) of light. Typical digital cameras have detectors sensitive to each of: red (R), green (G), and blue (B) light. Such cameras (“RGB digital cameras”) are incapable of reproducing hi-fidelity color images which are required in fields such as tele-dermatology. In addition, RGB digital cameras have a limited color gamut and cannot reproduce a full range of colors and shades. These inherent limitations are more pronounced when the surface of an object contains reddish colors.
Because of the limitations of RGB digital cameras, RGB digital cameras cannot be used where accurate measurement and/or reproduction of colors is required. For example, RGB digital cameras have limited application in color reproduction for tele-medicine, image archiving and follow-up, color matching and measurements, and tissue characterization and quantification in cancer characterization.
One method for obtaining accurate information about the color of a surface using a RGB digital camera is to use the camera to obtain images while illuminating the surface with spectral light at different wavelengths. The spectral light comprises a set of spectral bands, preferably narrow bands, centered on each of the different wavelengths. The spectral reflectance images are then acquired using an RGB camera. If the sensitivity of the sensors of the RGB camera at the wavelengths of the spectral light are known and the intensity of illumination in the different bands of the spectral light are known then the resulting set of images can be processed to determine the spectral reflectance of the surface.
This method is cost effective but requires a spectrometer and a dedicated lab facility to characterize the spectral sensitivity of digital camera sensors. It further requires pre-measurement calibration using a calibration target with a known reflectance at each wavelength, e.g. a white 99% diffuse reflectance disk. The pre-measurement calibration using a white diffuse reflectance disk ensures the effective spectral calibration of a camera sensor, especially when used to acquire spectral images in a non-lab-controlled environment.
There remains a need for digital cameras which can provide accurate spectral images of a target, even in dynamically changing viewing or illumination conditions. There is a particular need for such cameras that are cost effective enough for widespread application in fields such as tele-dermatology, color matching and the like.